1. Field of the Invention
This invention relates generally to fabrication methods for semiconductor devices.
2. Description of the Related Art
It is well known that a non-volatile semiconductor memory includes cell transistors, wherein the floating gate and control gate are stacked and bit information is stored by storing charge in the floating gate and changing the threshold value of the transistor. Flash memories are widely used non-volatile semiconductor memories which are electrically erasable in a block. There are NOR type, NAND type, AND type, and DINOR type flash memories, identified by the connection type of the cell transistors. With the increased capacity of flash memories in recent years, the NAND-type flash memories are generally employed for highly integrating cell transistors (Aritome, S, et. al. 1994 International Electron Devices Meeting Technical Digest, pp. 271-274, December 1994). Also, a virtual ground type array of SONOS cells in which the charge is trapped in the insulating film instead of the floating gate are also known (Sung et al.: Fabrication and Program/Erase Characteristics of 30-nm SONOS Nonvolatile Memory Devices, IEEE Transaction on Nanotechnology, vol. 2. no. 4, December 2003).
FIG. 1 is a plane view of a conventional NAND-type flash memory array. A plane pattern of the memory array includes a control gate 101 of a word line and a bit line 102, which are arranged to intersect each other. Floating gates 103 are provided in regions intersected by the aforementioned lines. An element isolation region 104 is provided to extend in a direction parallel to the word line 101 and located between the floating gates 103 so as to isolate the cell transistors arranged adjacently to each other. Shallow Trench Isolation (STI), utilized in miniaturized fabrication, is commonly employed for element isolation. An n-type diffusion layer to form the source/drain regions of the cell transistor is provided between the word lines 101 arranged adjacently to each other.
Therefore, the NAND-type flash memory has a string architecture in which the n-type transistors are connected in series and multiple strings are formed in parallel. One end of the NAND string is connected to a page buffer via a select transistor and the other end thereof is grounded to a common source region via another select transistor, not shown in FIG. 1 for simplification. In conventional fabrication methods, the line width and space of the STI, word line, and bit line are limited by a minimum process size of F, dependent on the photolithography characteristics. Hence, as shown in FIG. 1, F is the minimum size of the width and space respectively and 2F is the minimum pitch of the interconnection lines. Accordingly, 4F2 (2F×2F) is the minimum memory cell size.
Referring to FIGS. 2A to 2D, a description will be given of the conventional technique for processing a width narrower than the minimum process size of F. As shown in FIG. 2A, a silicon nitride film 106 is deposited on a base layer 105, and a mask pattern 107 of polysilicon is provided with a line width F and a space F. Next, as shown in FIG. 2B, after sidewalls 108 of silicon dioxide are formed on side walls, the mask pattern 107 is removed as shown in FIG. 2C. Then, the sidewalls 108 are used for the mask and the silicon nitride film 106 is etched. Subsequently, the sidewalls 108 are selectively removed, and the base layer 105 is etched with a silicon nitride film 106a used for the mask. As shown in FIG. 2D, a line pattern 105a with a width of x1, which is narrower than the minimum process size of F, is created, thereby forming two types of spaces between the lines, F and xs, where xs is narrower than F. The lines are arranged at an average pitch of F, however, the line spaces are not uniform.
It is to be noted that in conventional photolithography method with the minimum process size of F, F is the minimum width of the mask pattern used for forming the lines and F is the minimum space thereof. Therefore, it is impossible to accurately form lines haivng a width narrower than the aforementioned size with conventional fabrication techniques. In conventional techniques for forming the line pattern of the pitch F, the sidewall films formed on the side walls of the stripe pattern with the line width F and the space F are used for the mask, resulting in a constant line width. However, there is a problem in that the line space has two types. That is, it is difficult to make a high-density pattern with line width and line space of equal sizes. Accordingly, it is also difficult to reduce the costs of the NAND-type flash memory in which high-density cell arrays are demanded in view of the limitations on the process size due to the performance limitations of photolithography.